WO2009055710A2 - Strong and ductile low-field magnetostrictive alloys - Google Patents

Abstract

The present disclosure is directed to an energy conversion device that is adapted to convert magnetic energy into mechanical energy. The device includes a generator that is adapted to generate a magnetic field. The device also includes a body centered cubic crystalline alloy that can be subjected to the magnetic field generated by the magnetic field generator. The alloy has a room temperature saturation magnetostriction of at least 50 or 90 ppm along the [100] axis of the alloy crystals. The alloy includes a large atomic % of Fe. The alloy can also include Mo, W, or Cr. The alloy is adapted to convert magnetic energy into mechanical energy in the form of a change of dimension of the alloy when subjected to the magnetic field.

Description

STRONG AND DUCTILE LOW-FIELD MAGNETOSTRICTIVE ALLOYS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to United States provisional patent application no. 60/982,811 filed on October 26, 2007, entitled STRONG AND DUCTILE LOW-FIELD MEGNETO STRICTIVE ALLOYS, which is incorporated herein by reference in its entirety.

[0002] This application was made with government support under Grant #DMR0241603 awarded by the National Science Foundation. The Government has certain rights to this invention.

BACKGROUND

[0003] A magnetostrictive material is a material whose shape changes as a function of applied magnetic field. For example, if a rod of magnetostrictive material of length 1 is subject to an applied magnetic field H, its length will decrease or increase. The change in length Δl saturating at the value Al=X₅I, where λ_s is the magnetostrictive coefficient of the material, also referred to as the saturation magnetostriction. When a magnetic field is applied to a magnetostrictive material, it will expand or contract, such that a positive X₃ refers to expansion and a negative X₃ refers to contraction. The material currently having one of the largest known magnetostrictive coefficients is an alloy of terbium (Tb), dysprosium (Dy), and iron (Fe), commonly referred to as Terfenol, which possesses a λ_s>1000 ppm. A magnetostrictive coefficient in this range, often referred to as giant magnetostriction, may be useful for many technological applications, such as actuation and sensing. However, the Terfenol alloy is brittle and expensive. Additionally, the uses of the Terfenal alloy are limited to cryogenic temperature applications due to the Curie temperatures of TB and Dy, which are -184°C and -53⁰C, respectively. Moreover, the saturation magnetic field of TB and Dy is to high to be used in many sensor and actuator applications.

[0004] Significant effort has been devoted to the fabrication of composites of Terfenol and other materials, including polymers and metals, in an effort to combine the good magnetostrictive properties of Terfenol with a matrix material having more robust mechanical properties. In these composites, however, the magnetostrictive coefficient λ_s is often reduced by approximately the volume fraction of the magnetostrictive phase. For example, a composite containing 50% Terfenol often has a magnetostrictive coefficient that is approximately half that of bulk Terfenol. Additionally, many of these composites are expensive to manufacture. [0005] There is thus a need to provide a less expensive, less rigid magnetostrictive composite having large magnetostrictive properties and good mechanical properties, such that the material can be used in aggressive environments, such as automotive applications and the like.

BRIEF SUMMARY

[0006] The present invention is directed to an energy conversion device that is adapted to convert magnetic energy into mechanical energy. The device can include a body centered cubic crystalline alloy. In some exemplary embodiments, the alloy has a room temperature saturation magnetostriction of at least 50 ppm along the [100] axis of the alloy crystals. In other exemplary embodiments, the alloy has a room temperature saturation magnetostriction of at least 90 ppm along the [100] axis of the alloy crystals. The alloy includes a relatively large atomic % of Fe. Additionally, the alloy can also include one or more of Mo, W, and Cr. The device can include a magnetic field generator for generating a magnetic field to which the alloy is subjected. The magnetic field generator can include an electrically conducting coil that can be attached to, positioned around, or otherwise associated with the alloy. The magnetic field generator can also include a permanent magnet. The magnetic field to which the alloy is subjected can be varied by movement of the magnet relative to the alloy. When subjected to the magnetic field, the alloy is adapted to convert magnetic energy into mechanical energy in the form of a change of dimension of the alloy. [0007] This Summary is provided to introduce a brief selection of concepts in a simplified form. Particular embodiments of the present invention are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. [0008] Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In order to describe the manner in which the above-recited and other advantages and features of the invention can be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

[0010] Figure 1 illustrates a plot diagram of lattice parameters versus solute content of various Fe based alloys according to embodiments of the present invention;

[0011] Figure 2A illustrates the Fe-rich portion of the Fe-Mo phase diagram;

[0012] Figure 2B illustrates the Fe-rich portion of the Fe-W phase diagram;

[0013] Figure 3 A illustrates the X-ray diffraction pattern of a oriented Fe- 15 at.%

Mo single crystal using a Θ-2Θ scan;

[0014] Figure 3B illustrates the X-ray diffraction pattern of a oriented Fe-15 at.%

Mo single crystal using a (200) rocking curve scan;

[0015] Figure 4A illustrates the magnetostriction curves of a [100] oriented Fe-15 at.% Mo LTA single crystal with the strain measurement direction parallel to the

[100] crystallographic direction and the applied field direction along the [100] crystallographic direction;

[0016] Figure 4B illustrates the magnetostriction curves of a [100] oriented Fe-15 at.% Mo LTA single crystal with the strain measurement direction parallel to the

[100] crystallographic direction and the applied field direction along the [010] crystallographic direction;

[0017] Figure 5A illustrates the magnetostriction curves for a [100]-oriented Fe-

4.4 at.% W DG single crystal with the strain measurement direction parallel to the

[100] crystallographic direction and the applied field direction along the [100] crystallographic direction;

[0018] Figure 5B illustrates the magnetostriction curves for a [100] -oriented Fe-

4.4 at.% W DG single crystal with the strain measurement direction parallel to the [100] crystallographic direction and the applied field direction along the [010] crystal lographic direction;

[0019] Figure 6A illustrates the magnetostriction curves for a [100] -oriented Fe-

4.4 at.% W annealed single crystal with the strain measurement direction parallel to the [100] crystallographic direction and the applied field direction along the [100] crystallographic direction;

[0020] Figure 6B illustrates the magnetostriction curves for a [100]-oriented Fe-

4.4 at.% W annealed single crystal with the strain measurement direction parallel to the [100] crystallographic direction and the applied field direction along the [010] crystallographic direction;

[0021] Figure 7A illustrates the magnetostriction curves for a [100]-oriented Fe-

10 at.% W DG single crystal with the strain measurement direction parallel to the

[100] crystallographic direction and the applied field direction along the [100] crystallographic direction;

[0022] Figure 7B illustrates the magnetostriction curves for a [100]-oriented Fe-

10 at.% W DG single crystal with the strain measurement direction parallel to the

[100] crystallographic direction and the applied field direction along the [010] crystallographic direction;

[0023] Figure 8A illustrates the magnetostriction curves for a [100]-oriented Fe-

10 at.% W annealed single crystal with the strain measurement direction parallel to the [100] crystallographic direction and the applied field direction along the [100] crystallographic direction;

[0024] Figure 8BA illustrates the magnetostriction curves for a [100]-oriented Fe-

10 at.% W annealed single crystal with the strain measurement direction parallel to the [100] crystallographic direction and the applied field direction along the [010] crystallographic direction; and

[0025] Figure 9 illustrates the resonance spectra observed for an annealed Fe-4.4 at.% W alloy single crystal according to one embodiment of the present invention.

DETAILED DESCRIPTION

[0026] Magnetostrictive (or piezo-magnetic) materials exhibit reversible strains in response to an applied magnetic field and changes in magnetic properties such as permeability on the application of stress. Materials with large magnetostriction at low saturation fields, good mechanical properties, and low cost are beneficial in a wide range of actuator, sensor, and active device applications. Due to the relatively low cost, high magnetic moment, high strength and good ductility that can be obtained, Fe-based alloys in a bcc terminal solid solution phase without rare earth additions such as Tb and Dy can be particularly beneficial as potential low-field and low cost magnetostrictive alloys.

[0027] Of particular interest are iron-based alloys with alloying additives that are less expensive than rare earth elements. Based on the (i) atomic sizes relative to Fe, (ii) solubility limits in Fe, and (iii) ground state electronic structure, the choice of alloying elements to create such low-field magnetostrictive alloys can be narrowed to Ga, Be, AI, and Mo. See S. Guruswamy, N. Srisukhumbowornchai, A.E. Clark, J. Restorff and M. Wun-Fogle, Scripta Mater. 43, 239-244 (2000) (hereafter Guruswamy et al.); N. Srisukhumbowornchai and S. Guruswamy, J. Appl. Phys. 90, 5680-5688 (2001) (hereafter Srisukhumbowornchai et al.); and N. Srisukhumbowornchai, PhD dissertation, Univ. of Utah, Salt Lake City, UT, 2001 (hereafter Srisukhumbowornchai), each of which is incorporated herein by reference in its entirety. These elements have appreciable solubility in Fe and have in their ground state, either an unfilled d-shell (Be), a fully-filled d-shell (Al, Ga), or a half filled d-shell (Mo). Additions of Be and Al to Fe have been known to enhance magnetostriction in Fe. See R. Gersdorf, Magnetostriction of single crystals of iron and some dilute iron alloys, PhD dissertation, University of Amsterdam, Netherlands, 1961; Z.N. Bolycheva, M.M. Borodkina, and V.L. Sandomirskaya: Fiz. Metal. Metalloid, 19, 152-54 (1965); R. C. Hall, J. Appl. Phys. 28, 707 (1957); and A.E. Clark, M.W. Fogle, J.B. Restorff, T.A. Lograsso, and G. Petculescu, J. Appl. Phys. 95, 6942 (2004), each of which is incorporated herein by reference in its entirety. Similarly, the addition of Ga to Fe results in a dramatic increase in magnetostriction (from 30 x 1O^"6 to as high as 380 x 10^'6). See Guruswamy et al.; Srisukhumbowornchai et al.; Srisukhumbowornchai; N. Srisukhumbowornchai and S. Guruswamy, J. Appl. Phys. 92 5371-5379 (2002) (hereafter Srisukhumbowornchai and Guruswamy); T.V. Jayaraman, R.P. Corson, and S.Guruswamy, J. Appl. Phys. 102, 053905 (2007); P. Mungsantisuk, R.P. Corson and S. Guruswamy, J. Appl. Phys. 98, 123907-1 (2005); and P. Mungsantisuk, PhD Dissertation, University of Utah, Salt Lake City, UT, 2005, each of which is incorporated herein by reference in its entirety. This combined with low saturation magnetic fields of less than 100 Oe, high strength, good ductility, and high elastic moduli values make FeGa alloys very attractive in a number of industrial applications. This discovery also opens up the possibility of obtaining very large magnetostrictive strains in other low-cost ductile Fe-based alloys with no rare earth additions, as described herein. [0028] While the magnitude of the saturation strain in these alloys may be smaller than those in the Terfenol-D alloy and the Ni₂MnGa magnetic shape memory alloys, the saturation magnetostriction is obtained in Fe-Ga alloys at much lower applied magnetic fields. In some instances, the applied magnetic field is less than 100 Oe. Additionally, the saturation magnetostriction for Fe-Ga alloys can be obtained with low hysteresis (few Oe) relative to Terfenol-D alloys and Ni₂MnGa alloys with saturation fields well over 1000 Oe. Moreover, Fe-Ga alloys also exhibit an excellent combination of high strength, good ductility, and high elastic modulus that can be far superior to Terfenol-D magnetostrictive and Ni₂MnGa intermetallic alloys. With attractive mechanical properties and large saturation strains- of several hundred ppm, the Fe-Ga based alloys can now compete with piezoelectric materials and other magnetostrictive alloys, such as Terfenol-D, in a number of industrial applications. [0029] Described in greater detail below is the influence on the magnetostrictive behavior of Fe resulting from the addition to Fe of transition elements, such as Mo and W. Both Mo and W belong to Group IVB in the periodic table and have a half- filled d-shell in their ground state. A plot of the lattice parameter versus solute content for various binary alloys, as illustrated in Figure 1, shows that the variations of lattice parameters with solute content in Fe-Mo and Fe-W alloys are higher than that obtained with additions of Ga. The data plotted in Figure 1 was obtained from H. Okamoto, Phase Diagrams of Binary Iron Alloys, Monograph Series on Alloy Phase Diagrams No.9, edited by H. Okamoto, ASM International, Materials Park, OH, (1993), which is incorporated herein by reference in its entirety. [0030] In an effort to create highly magnetostrictive Fe based binary alloys, various elements were examined, including Be with no d-shell electrons, Ga with a fully-filled d-shell, and Mo with a half-filled d-shell. Using single crystal and polycrystalline Fe-Ga alloys, it has been shown that Ga dramatically increases the magnetostriction of Fe. Similarly, it has been shown that the addition of Be enhances the magnetostriction of Fe. See R. Gersdorf, Magnetostriction of Single Crystals of Iron and Some Dilute Iron Alloys, Dissertation, University of Amsterdam, Netherlands 1961; and A.E. Clark, M.W. Fogle, J.B. Restorff, T.A. Lograsso, and G. Petculescu, Magnetostriction and Elasticity of Body Centered Cubic Feioo-_x Be_x. J. Appl. Phys. 95 (2004) 6942, each of which is incorporated herein by reference in its entirety. The alloying of Fe with Group IVB elements was examined to determine how the addition of elements with a half-filled d-shell in the ground state influences the magnetostriction and elastic properties of Fe.

[0031] Of the three Group IVB elements Cr, Mo and W, Cr dissolves in α-Fe over the entire composition range at high temperatures. While there is a tendency upon slow cooling to form a sigma phase at intermediate Cr contents, this can be suppressed with rapid cooling from the high temperature region. On the other hand, the solubility limits of Mo and W in α-Fe are 24.4 at.% at 1449°C and 14.3 at.% at 1548°C, respectively. See A. Fernandez Guillermet, in Phase Diagrams of Binary Iron Alloys, Monograph Series on Alloy Phase Diagrams No.9, edited by H. Okamoto, ASM International, Materials Park, OR, (1993), pp.214-221; and S.V. Nagendar Naidu, A.M. Sriramamurthy, and P. Rama Rao, in Phase Diagrams of Binary Iron Alloys, Monograph Series on Alloy Phase Diagrams No.9, edited by H. Okamoto, ASM International, Materials Park, OR, (1993), pp. 444-453, each of which is incorporated herein by reference in its entirety. Solubility decreases to negligible levels at room temperature, but large amounts of solute can be retained in solution by rapid quenching from high temperatures. With increasing Mo or W content, the temperature range over which the disordered phase exists becomes narrow, and the disordered (α) phase can transform to various ordered phases upon cooling to lower temperatures as illustrated in Figure 2 A and 2B. However, the solid-liquid transition regions of Fe-x (x=Cr, Mo, W) alloys are small, which makes the single crystal growth easier as the constitutional supercooling effect is minimal. [0032] While the increase in λioo magnetostriction achieved due to an increase in Cr content is relatively small, the λioo magnetostriction of Fe-Mo alloys with 2.18, 3.41, 4.35, and 15 at.% Mo were 31.9, 32.5, 39.3xlO^~6, and 60 ppm, respectively. See R.C. Hall, Single-Crystal Magnetic Anisotropy and Magnetization Studies in Iron- Base Alloys, J. Appl. Phys. 31 (1960) 1037. A comparison of lattice parameters versus solute content in various Fe binary alloys (see Figure 1) shows that the addition of W results in a sharper increase in the lattice parameter as well as an increase in magnetostriction as one moves down the Group IVB column from Cr to Mo. This suggests that W can provide large magnetostriction. The Fe-rich regions of Fe-Mo and Fe-W phase diagrams show similarities though the W solubility in Fe less than that of Mo.

[0033] The results and process for determining the magnetostriction coefficients (3/2 λioo), the elastic constants (Cu, Cn, and C₄₄), and the magnetomechanical coefficients (Bj) for α-Fe-Mo and Fe-W alloy single crystals are described in detail below. Briefly, single crystals of Fe-Mo and Fe-W alloys were grown using the vertical Bridgman technique. Magnetostriction measurements were carried out along [100] crystal lographic directions, and (3/2) λioo values were calculated. Elastic constants Cy were estimated using the resonant ultrasound spectroscopy (RUS) technique. Using the elastic constants Cy, other elastic properties such as anisotropy ratio (A), bulk modulus (B), and Young's modulus (Ep_1Id]) were calculated. In addition, Young's modulus and Poisson ratio measurements were also made using compression tests in an Instron universal testing machine and compared with the results from the RUS measurements. The magneto-elastic coupling constant (B ϊ) was calculated from the elastic constants and the magnetostriction coefficient. All the measurements were performed at room temperature. PREPARATION OF ALLOYS AND SINGLE CRYSTALS

[0034] In one exemplary embodiment of the present invention, Fe-based alloys crystals can be prepared as described below. It will be appreciated by one of ordinary skill in the art, however, that other techniques and processes can be employed to form the Fe-based alloys and crystals of the present invention. Throughout this description the standard notations will be used to refer to various elements and attributes of the alloys and crystals described herein. By way of example, elements will be referred to using the following symbols: Fe (iron), W (tungsten), Mo (molybdenum), Cr (chromium), Ga (gallium). Similarly, standard notations, such as at.% for atomic percentage, will also be used herein.

[0035] Ingots of Fe- 15 at.% Mo, Fe- 17.5 at.% Mo, Fe-4.4 at.% W, Fe-7.5 at.% W, and Fe-IO at.% W alloys can be prepared using high-purity elements in an EDMUND BUHLER^® high vacuum arc-melting system, as is well known in the art. The ingots can be remelted several times to assure homogeneity. The ingots can then be cast into a rod form by allowing the liquid metal to flow into a 12.5 mm diameter cylindrical cavity inside a copper block with a thick alumina insulation sleeve. The Fe-15 at.% Mo, Fe-17.5 at.% Mo, Fe-4.4 at.% W, Fe-7.5 at.% W, and Fe-IO at.% W alloy single- crystal rods can be obtained using the vertical Bridgman technique. The arc-melted and cast alloy rods are placed in a long closed-one-end alumina tube and the open end of the tube is then connected to an evacuation and purging line. The alumina tube is then positioned at the middle of the furnace so that all the molten material will initially be in the region of the maximum set temperature. After repeated evacuation and ultra-high-purity (UHP) argon gas purging, the tube is heated in a resistance- heated 1700°C 2-zone tube furnace to the desired temperature. Argon gas flow is maintained during the entire process. The tube is moved down the temperature gradient in the furnace at a controlled rate in the range of ~4 to 22.5 mm/h using a stepper-motor-drive as is known in the art. As the tube is moved down the temperature gradient, the solidification of the melt starts from the bottom end of tube and directionally grown (DG) Fe-Mo and Fe-W alloy rods containing a single large crystal or a few large single crystals are obtained.

X-RAY DIFFRACTION CHARACTERIZATION OF SINGLE CRYSTAL ALLOY SAMPLES [0036] Single crystals can be cut from the DG rods and characterized using a SIEMENS^® D5000 high-resolution x-ray diffractometer and Cu K_α radiation. The θ- 2Θ x-ray diffraction scans illustrated in Figure 3A can be used to determine the 2-theta value corresponding to the (002) peak for use in setting up the rocking curve scans illustrated in Figure 3B. Rocking curves in conjunction with the detector and phi scans can be used for single-crystal orientation determination. The resolutions that can be obtained in the rocking curve scans are better than 0.01 degrees. This information can be used for cutting and polishing the samples to the desired crystallographic direction orientation. Parallelepiped-shaped single crystal samples, typically about 11x7x5 mm in size and oriented to within 0.5 degrees off the desired [100] crystallographic orientation, can be prepared for magnetostriction and elastic moduli measurements as is known in the art. Disc samples with the disc surface normal to the [110] crystallographic direction can also be prepared. MAGNETOSTRICTION AND MAGNETIZATION MEASUREMENTS

[0037] The magnetostriction of the parallelepiped single crystal samples can be measured by attaching a strain gage on the flat face of the crystal. The full bridge strain gage technique can be used with an active strain gage attached to the sample and three dummy gages attached to a Bi rod, which has a coefficient of thermal expansion value similar to that of Fe. A coil surrounding the sample can be used to apply the magnetic field, which can be varied between -800 to 800 Oe. The presence of Fe end pieces ensures that the magnetization is uniform along the entire length of the sample. The saturation magnetostriction measured along the single crystal sample axis ([100] or [111] in this study) with the applied magnetic field oriented parallel to sample axis is referred to as λn. Similarly, the saturation magnetostriction measured along the single crystal sample axis with the applied magnetic field oriented perpendicular to the sample axis is referred to as λ±. The λn and λi measurements are used to obtain the single crystal magnetostriction constants (3/2) λioo and (3/2) λm. Signals corresponding to the applied magnetic filed and magnetostrictive strains can be collected using an IOTECH data acquisition system.

[0038] The magnetic properties at room temperature can be determined using a LAKESHORE^® Model 7307 vibrating sample magnetometer (VSM). The samples have a cubic shape and are approximately 2x2x2 mm in size. The maximum applied magnetic field is 10 kθe.

ELASTIC CONSTANT MEASUREMENTS

[0039] The elastic moduli of the various alloy single crystals can be measured using the resonance ultrasound spectroscopy (RUS) technique. The samples with a parallelepiped shape are gently held at diametrically opposite corners between a pair of broadband piezoelectric ultrasonic transducer (the transmitter and the receiver) faces, such as those of a MODULUS II^® Dynamic Resonance System (DRS). Ultrasonic signals of constant amplitude with frequencies varying from 50 to 900 kHz can be transmitted through the sample. The signals detected at the receiver provide an absorption spectrum with sharp peaks at frequencies corresponding to the sample's resonant frequencies.

[0040] The sample faces are carefully hand polished using a wedge polishing fixture to achieve the sample axis orientation that is within 0.5° from the desired crystal axis. All the faces are polished down to a 3-μm diamond finish while maintaining sharp edges and corners by avoiding chipping or cracking. The final dimensions of the RUS samples depend on the size of the single crystal available for a given alloy.

[0041] In one exemplary embodiment, the elastic constants for the crystals can be determined through multiple resonance spectra measurements. For example, eight independent measurements of resonance spectra can be made and the first 40 resonance peaks can be used in the computation of elastic constants. The computation process involves an iterative process of fitting experimental spectra with a theoretically calculated list of resonances based on the given sample parameters, such as weight, length, width, and thickness, and iteratively adjusted elastic constant values. See A.Migliori, and J. L. Sarrao, Resonant Ultrasound Spectroscopy: Applications to Physics, Materials Measurements and Non-Destructive Evaluation, (John Wiley & Sons, New York, 1997); J. B.Zadler, H.L. Jerome, Le Rousseau, A. John, Scales and M. L. Smith, Geophysics. J. Int. 156, 154-169 (2004); R. G. Leisure and F. A. Willis, Journal of Physics: Condensed Matter 9, 6001-6029, (1997); and Dynamic Resonant Systems (DRS) Inc. Users Guide for DRS RUS Modulus II^® System, each of which is incorporated herein by reference in its entirety. The theoretically computed spectra can be closely matched to the experimental input with a root-mean-square (rms) deviation of « 0.2%. RMS values of less than 0.2% provide reliable data with an accuracy that is better than 1x10^"3. The anisotropy ratio (A) and elastic moduli values can be calculated from the elastic constants Cn, C]₂, and C₄₄ determined using the RUS analysis technique. See J. P. Hirth and J. Lothe, Theory of Dislocations, Second edition, (John Wiley & Sons Inc., New York, 1982); Kelly, G. W. Grooves and P. Kidd, Crystallography and Crystal Defects, Revised second edition (John Wiley & Sons Inc., New York 2000); T. H. Courtney, Mechanical Behavior of Materials, 2^nd ed., McGraw Hill series in Materials Science and Engineering, (McGraw Hill, Boston, 2000); R.E. Newnham, Structure-Property Relations (Springer-Verlag Berlin, Germany 1975); M. Jain and M.P. Verma, Indian Journal of Pure and Applied Physics 28, 178 (1990); R.H. Baughman, J.M. Shacklette, A.A. Zakhidov, and S. Stafstrom, Nature 392, 362 (1998); K.E. Evans and M.A. Nkansah, Nature, 353, 124 (1991); and K.E. Evans and K.L. Alderson, Engineering Science and Education Journal 9, 148 (2000), each of which is incorporated herein by reference in its entirety.

[0042] The parameter {C _U-Ci₂)Il represents the resistance to shearing on the {010} planes in the <110> directions and this should be equivalent to the constant C₄₄ that represents the resistance to shear on the { 100} planes in the <100> type directions. See Courtney. The anisotropy factor A is the ratio between shear stress in these two cases and defined as:

1C A = _(r " _Λ (Equation 1)

If the material is isotropic, the anisotropy ratio should be equal to one. If A < 1, then the crystal is stiffest along the <100> cube edges and most compliant along the <111> body diagonals. However, if A > 1, the crystal is more compliant along the <100> and stiffer along the <111>. See R.E. Newnham.

[0043] Young's moduli of a single crystal depend on the direction of the applied stress, and, for an arbitrary [hkl] direction, can be calculated using the following equation:

1 ^■ = S r,_u -2 ~(,Sπ_u -S r,_l2 -- 1 S πJ w(a 2²βn2² + , a „,Y2. ,2 + , β oY2.,2>) ^*'^] (Equation 2) where a, β and γ are the direction cosines of the [hkl] direction with respect to the [100], [010] and [001] directions, respectively, and S₁₁, Su and S₄4 are compliances related to the elastic constants C₁₁, C₁₂ and C ₄4 as given by the following equations:

5₁₁ = (C_/; + C₁₂) I ((C₁₁- C₁₂) (C₁₁ + 2C₁₂)) (Equation 3)

5₁₂ = - (C₁₂) I ((C₁₁- C₁₂) (C₁₁ + 2C₇₂)) (Equation 4) S₄₄ = I/C₄₄ (Equation 5)

[0044] Poisson's ratio is the negative ratio of the transverse and the longitudinal strain. In single crystals, two independent Poisson's ratios corresponding to two lateral directions that are perpendicular to each other can be defined for every loading direction. See M. Jain and M.P. Verma. For the loads applied along the [100] direction, Poisson's ratio can be calculated according to:

C₁₂ v[ioo,oio] = ^v[ioo,ooi] = _{(r r Λ} (Equation 7)

where the suffixes corresponding to the Poisson ratio term indicate the loading and the transverse strain measurement directions and the Cy' s are the elastic constants of

the crystal. In case of a [1 10] loading direction, Poisson's ratios for both [1 1 0] and [001] strain measurement directions are expressed by the following equations: (Equation 8)

^[110-^110] (R + 2C₄₄)

4C C

"[110.001] = " " , (Equation 9)

C₁₁CK + 2C₄₄)

Where

Λ = C₁₁ (Equation 10)

See R.H. Baughman, J.M. Shacklette, A.A. Zakhidov, and S. Stafstrom.

MAGNETOSTRICTION IN FE-(W, MO) BASED ALLOY SINGLE CRYSTALS [0045] X-ray diffraction patterns corresponding to the rocking curve and Θ-2Θ scans can be obtained for all the six faces of the [100] Fe-15 at.% Mo, Fe-17.5 at.% Mo, Fe-4.4 at.% W, Fe-7.5 at.% W, and Fe-IO at.% W single crystal samples (both DG and annealed). Figure 3A shows the diffraction pattern obtained from a Θ-2Θ scan of the (lOO)-face of the [100] -oriented and annealed Fe-15 at.% Mo single crystal. Figure 3B shows the corresponding (200) rocking curve for the (lOO)-face of the [100]-oriented and annealed Fe-15 at.% Mo single crystal. Besides the fundamental (200) peak, the Θ-2Θ scan (Figure 3A) also shows a diffuse scattering peak in the low angle region between 20 and 40 degrees indicating the presence of a short range order in the sample. The diffuse peak observed is more pronounced in the case of a Fe-4.4 at.% W alloy single crystal, thus suggesting a higher level of short range ordering in this alloy.

[0046] The (200) rocking curve scan of [100]-oriented Fe-15 at.% Mo single crystal (Figure 3B) shows a ω value of 31.45°. Using this data along with the measured 2Θ value of 63.825 degrees for the (200) peak from Θ-2Θ scan (Figure 3A), the crystal orientation can be determined to be within 0.5 degrees of the [100] crystallographic axis. For each of the alloys described herein, the orientations of the single crystal sample faces are within 0.5 degrees of the <100> direction. [0047] The (3/2)λioo values obtained from the various Fe-Mo and Fe-W alloy single crystals are presented in Table I. For Fe-Mo alloys, the saturation magnetostriction values are reached at fields of less than 100 Oe. The magnetostrictions measured along the [100] direction with the magnetic field applied parallel and perpendicular to the [100] direction of the long-term annealed (LTA) Fe- 15 at.% Mo alloy single crystal are shown in Figures 4A and 4B, respectively. The temperature and duration of the LTA treatment for the Fe- 15 at.% Mo alloy single crystal were 1250 ⁰C and 70 days. The saturation magnetostriction values observed in the annealed Fe-15 at.% Mo single crystal sample under no applied load condition are +102xl0^"6 and -2IxIO^"6 when the magnetic field is applied parallel and perpendicular to the [001] direction, as illustrated in Figures 4A and 4B, respectively. The (3/2)λioo obtained from the difference between these two measurements for Fe- 15 at.% Mo single crystal is 133 x 10^"6, more than three times the value observed in Fe-4.35 at.% Mo alloy.

Table I. Magnetostriction and saturation magnetization data for Fe-X (X= Mo or W) alloy single crystals.

Fe-15 at.% Mo Annealed at 1250 °C-70 123 21 164 days

Fe-17.5 at. % Mo DG* 24 103 100

Fe-17.5 at. % Mo Annealed at 1400 °C-4 h 75 0.1 ** 155

Fe-4.4 at. % W DG 97 21 181

Fe-4.4 at. % W Annealed at 1400 °C-4 h 97 0.3*** 179

Fe-7.5 at. % W DG 93 22 170

Fe- 10 at. % W DG 99 26 158

Fe- 10 at. % W Annealed at 1500 °C-2 h 122 0 9*** 156

* DG refers to as directionally-grown single crystal sample

** (100) orientated VSM sample (~7.4xl.3xl .3 mm in dimension) magnetized along [100] direction

***(100) orientated thin disk VSM sample (-7.2x4.5x0.5 mm in dimension) magnetized along [010] direction. Other VSM samples of this set were 2x2x2 mm in dimension magnetized and saturated in a random direction

[0048] The Fe-Mo phase diagram shows that several ordered phases (R, μ, and λ, which have prototype i?-(Co,Cr,Mo), Fe₇W₆, and MgZn₂, respectively) can form on equilibrium cooling of the disordered bcc phase of Fe-Mo alloys with high Mo content (Figure 2). For the Fe-17.5 at.% Mo alloy, the disordered phase transforms to ordered phases upon cooling. The R phase first appears as a result of the peritectic reaction at temperatures below 1300 ⁰C. During the DG processing of Fe-Mo alloys with higher Mo content, it can be difficult to ensure that the alloy is not exposed appreciably to temperatures within the ordered region because of the narrow temperature range of the alpha phase region. To eliminate or minimize the presence of the ordered phase, the Fe-17.5 at.% Mo single crystals can be given a short-term solution annealing treatment at 1400 ⁰C for 4 hours and then quenched in water. The (3/2)λioo values of the Fe-17.5 at.% Mo single crystals can be measured both before and after annealing and compared. Before annealing, the (3/2)λioo of the Fe-17.5 at.% Mo is 24 χlθ^"6 (Table I). The magnetostriction value observed in the Fe-17.5 at.% Mo alloy single crystal after the short-term annealing treatment is 75 xlO^"6, which is greater than that observed in the DG sample but still lower than that observed in the Fe- 15 at.% Mo single crystal sample. As discussed below, coherent second phases are likely present in these samples and the lower magnetostriction values observed may be attributed to the presence of these second phases.

[0049] Table I also presents the saturation magnetization values of the Fe-Mo and Fe-W alloy single crystals measured using a VSM. The data clearly shows the drastic reduction in saturation magnetization in the Fe-17.5 at.% Mo DG single crystal from that of the Fe- 15 at.% Mo LTA single crystal. This reduction in saturation magnetization in the Fe-17.5 at.% Mo DG alloy sample suggests the presence of nonmagnetic second phase particles as the nonmagnetic phases reduce the amount of Fe atoms in the magnetic matrix phase. See LJ. Dykstra, Nonferromagnetic Precipitate in a Ferromagnetic Matrix, in Magnetism and Metallurgy vol. 2., A.E. Berkowitz and E. Kneller, ed. (Academic Press, New York 1969); S. Mishra, P.A. Beck, Atomic Order-Disorder Information From Magnetic Data, in Order-Disorder Transformations in Alloys, H. Warlimont, editor, (Springer- Verlag, New York, 1974), p 344; and B.D. Cullity, Introduction to Magnetic Materials (Addison-Wesley, Reading MA, 1972), each of which is incorporated herein by reference in its entirety. [0050] The pinning of the domain by the nonmagnetic second phase particles leads to a higher coercivity of 103 Oe observed in the Fe-17.5 at.% Mo in DG alloy sample compared to about 20 Oe observed in other alloy samples in this group with similar geometry. After annealing at 1400 ⁰C and water quenching, the saturation magnetization value for the Fe-17.5 at.% Mo DG single crystal increases to 155 emu/g. The changes in coercivity and magnetization values in the Fe-17.5 at.% Mo DG alloy clearly suggest the formation of a nonmagnetic second phase, and the drastic reduction in magnetostriction in this alloy results from the formation of the second phase.

[0051] The saturation magnetostriction values measured in a parallepiped single crystal sample of the Fe-4.4 at.% W DG alloy under no applied-load conditions were +9.4XlO^"6 and -88.5xlO^"6 with an applied magnetic field parallel and perpendicular to the [100] direction, respectively, as illustrated in Figures 5 A and 5B. The (3/2)λioo obtained from these measurements for the Fe-4.4 at.% W DG single crystal sample is 98XlO^"6. The Fe-4.4 at.% W DG single crystal alloy sample can then be annealed at 1400° C for 4 hours. From the measurements of magnetostriction with the magnetic field applied parallel and perpendicular to the [100] direction, the (3/2)λioo value for the Fe-4.4 at.% W single crystal after short-term annealing and quenching can be determined to be 98x10^"6, as shown in Figure 6A. This value is almost the same as that observed in the Fe-4.4 at.% W DG alloy single crystal. No significant difference in the saturation magnetization value is observed between the DG sample and the annealed and quenched sample, thus suggesting a less pronounced tendency to form ordered phases in the Fe-4.4 at.% W single crystals in both the DG and annealed conditions.

[0052] Magnetostriction in the [111] direction can be measured using a [HO]- oriented DG Fe-4.4 at.% W alloy single crystal disk, with a diameter of 12.5 mm and a thickness of about 2mm. The strain can be measured in the [111] direction and the magnetic field can be applied both parallel and perpendicular to the [1 1 1] direction. When the magnetic field is applied parallel to [11 1] direction, the saturation magnetostriction can be observed to be -lβxlO^"6. Similarly, when the magnetic field is applied parallel to [110] direction, the saturation magnetostriction is 1OxIO^"6. The difference between these two values provides a (3/2)λm value for this sample of -26xlO^"6.

[0053] The (3/2)λioo for the Fe- 10 at.% W DG sample can be observed to be 99.5 x 10^"6, which is nearly the same as that observed in the Fe-4.4 at.% W single crystals, as shown in Figure 7A. The Fe- 10 at.% W DG single crystal sample can be given a short-term annealing at 1500° C for 2 hour and then quenched in water to suppress the ordered second phase formation and retain the disordered phase at room temperature. Figures 8A and 8B show the magnetostriction versus applied field curves for [100]-oriented Fe- 10 at.% W single crystals after short term annealing at 1500° C for 2 hours when the magnetic field is applied parallel and perpendicular to the [100] direction. The (3/2)λioo value for the Fe-10 at.% W single crystal increases to 122xlO^"6 after short term annealing, a 20% percent increase from that of the DG sample and almost the same value as that observed in the Fe- 15 at.% Mo single crystal. No significant difference in saturation magnetization values is observed between the DG and annealed samples, suggesting a less pronounced tendency to form ordered phases in the Fe-IO at.% W single crystals.

ELASTIC MODULUS MEASUREMENT

[0054] Analyzing the RUS spectra of respective samples, the elastic moduli values for Fe- 15 at.% Mo, Fe-4.4 at.% W, and Fe-IO at.% W alloy single crystals in the annealed condition can be obtained. Table II summarizes the elastic moduli values for Fe- 15 at.% Mo, Fe-4.4 at.% W, and Fe-IO at.% W. Additionally, Figure 9 shows portions of the RUS spectra of the Fe- 15 at.% Mo and Fe-4.4 at.% W single crystals. Because the spectra show sharp peaks, the peak positions are easy to determine with high accuracy. The rms errors for the fitting of all 40 resonance peaks in the range of 80-700 kHz can be less than 0.08%, thereby showing an excellent agreement between the measured and calculated resonances. The anisotropic elastic constant values, C₁₁, C]₂, and C₄₄ for all the samples can be obtained from the theoretical fit of these curves. The anisotropic elastic constant values, Cu, C ₁₂, and C ₄₄ for all the samples are summarized in Table II. See G. E. Dieter, Mechanical Metallurgy (McGraw-Hill, New York, 1988), which is incorporated herein by reference in its entirety. These constants can be used to determine the anisotropy ratio and Young's moduli values using Equations 1-2 and 7-9, respectively. Additionally, the results can be compared to the elastic constants Cu, Cn, and C ₄₄ of pure iron.

Table II. Elastic constant, Young's moduli, and anisotropy constant data for Fe-X (X= Mo or W) alloy single crystals.

Elastic constant (GPa) Young's Moduli (GPa) Anisotropy Sample Constant,

Cn Ci2 C44 Eioo Eiio Em A

^Fe 237 141 116 131.81 220.58 284.43 2.42

Fe-15 Mo 255.3 158.4 111.5 134.00 220.03 279.94 2.30

Fe-4.4 W 245 146.2 117.9 135.72 225.85 290.06 2.38

Fe-IO W 251.8 152.2 119.8 137.12 229.38 295.70 2.40

[0055] For the bcc structures, the [111] direction is a close-packed direction and contains the greatest linear atomic density. When the stresses are applied along the [1 1 1] direction, the bcc crystals are stretched along the [1 1 1] direction without bending, which leads to the highest stiffness in this direction. In contrast, an applied stress in the [100] direction can rotate and elongate the near-neighbor bonds following a zigzag path in the [100] direction. See R.E. Newnham. Therefore, the stiffness coefficient in this direction is less than that in the [111] direction. Based on this, it can be determined that Eioo< Ei /o< Em for the bcc structures. This can be confirmed by the comparison of Young's moduli of alloys in Table II. The anisotropy factor Λ of pure iron is 2.4. The addition of Mo and W decreases the anisotropic factor of Fe, and therefore the crystal is less compliant along the [100] direction and stiffer along the [111] direction as a result of the decrease of Em and the increase of Em- [0056] Similarly, Poisson's ratios for the [100] and [110] loading directions can be calculated using Equations 7-9. Table III shows that pure iron, Fe-Mo and Fe-W alloys have negative Poisson's ratio v ^ values, indicating auxetic behavior. The

v_[ioo_,ooi_] value, corresponding to the [100] loading direction for all alloys discussed herein, has almost the same value as that of pure Fe. On the other hand, the v_r tj_ui_nu, ii_nu_nu}. ^and v (.110,11 ■0} values for the [110] loading σ direction varied from that of p i ure Fe.

Table III. Poisson's ratios of annealed [001] -oriented Fe-X

(X= Mo or W) alloy single crystals for the [100] and [110] loading directions.

[100] [110]

_, , loading loading Sample —

V[IOO₁ OOl] V[I iO₁ OOl] ^V[110,1ΪO]

Fe 0.37 0.624 -0.049

Fe-15 at.% Mo 0.38 0.629 -0.013

Fe-4.4 at.% W 0.37 0.622 -0.042

Fe-10 at.% W 0.38 0.630 -0.043

[0057] The magnetoelastic coupling allows the energy from the elastic system to be transferred to the magnetic system and vice versa. The magnetoelastic coupling constant B_/ provides a measure of this coupling. The relation between magnetostriction, elastic constant, and magnetoelastic coupling can be expressed by Equation 10. See A.E. Clark, and K. Hathaway, Handbook of Giant Magnetostrictive Materials (Academic Press, San Diego CA, 2002), which is incorporated herein by reference in its entirety.

(3 / 2M₁₀₀ = -J?, CC₁ , - C₁₂ ) (Equation 10)

[0058] Substituting the values for (3/2)λioo, Cn, and Cn, B/ for single crystals of each alloy described herein can be obtained. The Bi values are summarized in Table IV. Fe- 15 at.% Mo and Fe-IO at.% W have the highest magnetostriction B₁ values of all the alloys discussed herein.

Table IV. Magnetostriction and magnetoelastic coupling constants in annealed Fe-Mo and Fe-W alloy single crystals.

Sample Magnetostriction Magnetoelastic constant coupling constant,

3/2 λioo B₁ (MPa) (x 10-⁶)

Fe-15 at.% Mo 123 -11.92

Fe-4.4 at.% W 98 -9.68

Fe-IO at.% W 122 -12.18

[0059] The above discussion illustrates that the addition of Mo and/or W to Fe significantly enhances the magnetostriction of Fe. The (3/2)λio_O magnetostriction values measured using [100] -oriented annealed Fe-15 at.% Mo, Fe 4.4 at.% W, and Fe-IO at.% W single crystals are 123x lO^"6, 98χ lO^'6, and 122χ lO^"6, respectively. On a per atom addition basis, the addition of W to Fe provides a larger increase in magnetostriction compared to any other non-rare-earth addition that has been examined with the exception of Ga. The Fe-Mo and Fe-W alloys are attractive due to their large low field magnetostriction, high saturation magnetic moments, low coercivities, high elastic modulus, good ductility, high strength, and low cost. [0060] Although particular embodiments are described herein, the present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

CLAIMSWhat is claimed is:

1. An energy conversion device for converting magnetic energy into mechanical energy, comprising: a magnetic field generator; and a body centered cubic crystalline alloy adapted to be subjected to a magnetic field generated by said magnetic field generator, said alloy having a room temperature saturation magnetostriction of at least about 50 ppm along the [100] axis of the crystals constituting the alloy, the alloy comprising Fe and W; wherein said alloy is adapted to convert magnetic energy into mechanical energy in the form of a change of dimension of said alloy.

2. The device of claim 1, wherein said alloy comprises about 85 atomic % Fe to about 99 atomic % Fe and about 1 atomic % W to about 15 atomic % W.

3. The device of claim 2, wherein the alloy comprises about 90 atomic % to about 96 atomic % Fe and about 4 atomic % to about 10 atomic % W.

4. The device of claim 3, wherein said alloy comprises about 90 atomic % Fe and about 10 atomic % W.

5. The device of claim 1, wherein said alloy further comprises Mo or Cr.

6. The device of claim 1, wherein said alloy is a single crystal.

7. The device of claim 1, wherein said alloy is a polycrystalline alloy.

8. The device of claim 1, wherein said magnetic field generator comprises an electrically conducting coil associated with said alloy.

9. The device of claim 1, wherein said magnetic field generator comprises a permanent magnet and the magnetic field varies by the movement of the magnet relative to said alloy.

10. An energy conversion device for converting magnetic energy into mechanical energy, comprising: a magnetic field generator; and a body centered cubic crystalline alloy adapted to be subjected to a magnetic field generated by said magnetic field generator, said alloy having a room temperature saturation magnetostriction of at least about 50 ppm along the [100] axis of the crystals constituting the alloy, said alloy comprising Fe and Mo; wherein said alloy is adapted to convert magnetic energy into mechanical energy in the form of a change of dimension of said alloy.

11. The device of claim 10, wherein said alloy comprises about 80 atomic % Fe to about 90 atomic % Fe and about 10 atomic % Mo to about 20 atomic % Mo.

12. The device of claim 11, wherein said alloy comprises about 85 atomic % Fe and about 15 atomic % Mo.

13. The device of claim 10, wherein said alloy further comprises W or Cr.

14. The device of claim 10, wherein said alloy is a polycrystalline alloy.

15. The device of claim 10, wherein said magnetic field generator comprises an electrically conducting coil associated with said alloy.

16. The device of claim 10, wherein said magnetic field generator comprises a permanent magnet and the magnetic field varies by the movement of the magnet relative to said alloy.

17. A device for converting magnetic energy into mechanical energy, comprising: a magnetic field generator; and a body centered cubic crystalline alloy subject to a magnetic field generated by said magnetic field generator, said alloy having a room temperature saturation magnetostriction of at least about 90 ppm along the [100] axis of the crystals constituting the alloy, the alloy comprising Fe and W; wherein said alloy converts magnetic energy into mechanical energy in the form of a change of dimension of said alloy.

18. The device of claim 17, wherein said alloy comprises about 90 atomic % Fe to about 99atomic % Fe and about 1 atomic % W to about 10 atomic % W.

19. The device of claim 17, wherein said alloy comprises about 90 atomic % to about 96 atomic % Fe and about 4 atomic % to about 10 atomic % W

20. The device of claim 17, wherein said alloy further comprises Mo or Cr.

21. The device of claim 17, wherein said alloy is a single crystal.

22. The device of claim 17, wherein said magnetic field generator comprises an electrically conducting coil associated with said alloy.

23. The device of claim 17, wherein said magnetic field generator comprises a permanent magnet and the magnetic field varies by the movement of the magnet relative to said alloy.

24. An energy conversion device for converting magnetic energy into mechanical energy, comprising: a magnetic field generator; and a body centered cubic crystalline alloy subject to a magnetic field generated by said magnetic field generator, said alloy having a room temperature saturation magnetostriction of at least about 90 ppm along the [100] axis of the crystals constituting the alloy, said alloy comprising Fe and Mo; wherein said alloy converts magnetic energy into mechanical energy in the form of a change of dimension of said alloy.

25. The device of claim 24, wherein said alloy comprises about 80 atomic % Fe to about 90 atomic % Fe and about 10 atomic % Mo to about 20 atomic % Mo.

26. The device of claim 24, wherein said alloy further comprises W or Cr.

27. The device of claim 24, wherein said alloy is a single crystal.

28. The device of claim 24, wherein said alloy is a polycrystalline alloy.

29. A method of producing at least one [100] textured polycrystalline alloy by thermo-mechanical processing that involves rolling and annealing, wherein the polycrystalline alloy has a room temperature saturation magnetostriction of at least about 50 ppm and comprises Fe and at least one of Mo, W, and Cr.

30. The method of claim 29, wherein said alloy further comprises an amount of a texturing agent effective to control grain growth and favor development of a [001] or [100] orientation.